Child friendly tips:
Skip the paper liners. Use baking spray to make the muffin tins nonstick. Paper just gets in the way, even for experienced bakers.
Use a small "ice cream scoop" to measure batter for each muffin. A small scoop will allow for 2 scoops and you can pop the jam in between scoops.
Make an apron by using an adult or older sibling's old shirt.